Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The starter/controller, referred to hereinafter as simply a controller, uses an SCR, such as a triac, for controlling application of AC line voltage to the motor. The controller typically includes heat sinks associated with the SCRs for dissipating heat. When the controller has been running for an extended period of time, the heat sinks and SCRs become warm. If the motor stalls, then the SCRs must carry the motor stall current until protective overload relays trip to stop operation. The size of both the SCR and heat sink must become larger as the test current during stall is increased. In fact, when testing to IEC 947-4-2 standards, the prospective test current is 7.2 times the operational rating of the controller.
Currently controllers must be provided with larger SCRs and/or heat sinks to pass the tests. Alternatively, a current sensor must be used to limit the current during a stalled motor condition. This can increase the cost of the product.
These problems exist because the overload capacity of the overload relay may not match the overload capacity of the controller. At stall currents the overload capacity of the overload relay decreases more slowly than that of the controller. Thus, the controller may not be able to withstand the overload condition of the IEC test for the time that is required for the overload relay to trip.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.